Field of the Invention
The present invention relates to pixel driving of an organic electro-luminescence display device, and particularly, to a pixel driving apparatus of an organic electro-luminescence display device capable of avoiding a degradation of image quality due to a deterioration of a driving apparatus of an organic electro-luminescence display device.
Discussion of the Related Art
In general, an organic electro-luminescence display device is one type of flat panel display device. When a voltage is applied to two electrodes which face each other with an organic emitting layer interposed therebetween, electrons injected from one electrode and holes injected from the other electrode form pairs in the organic light emitting layer. Accordingly, luminescent molecules of the organic light emitting layer are excited to thereafter return to a ground state, thus to create energy, and such energy is emitted by the organic electro-luminescence display device. The organic electro-luminescence display device capable of emitting light as mentioned above attracts attention as a next generation display device because of its high visibility, light weight, thin configuration, and low voltage driving.
Depending on an existence of a switching device disposed in a unit pixel on an organic luminescence display panel, the organic electro-luminescence display device may be divided into an active-matrix type organic electro-luminescence display device and a passive-matrix type organic electro-luminescence display device.
FIG. 1 is a block diagram of an organic electro-luminescence display device according to the related art. As shown in FIG. 1, the organic electro-luminescence display device includes a display controller 10 for generating first and second timing signals TS1 and TS2 by receiving original video data from the exterior and a control signal for the data so as to output the first timing signal TS1 and the image signal DATA to a data driving unit 20 and output the second timing signal TS2 to a gate driving unit 30, the data driving unit 20 for outputting data voltages to data lines D1˜Dm on an organic electro-luminescence display panel 40, responsive to the image signal DATA inputted from the display controller 10, the gate driving unit 30 for receiving the second timing signal TS2 from the display controller 10 to sequentially output scan signals for driving scan lines S1˜Sn on the organic electro-luminescence display panel 40, and the organic electro-luminescence display panel 40 having OLED pixels PX arranged in a matrix at intersections between the scan lines S1˜Sn and the data lines D1˜Dm.
The pixels of the active-matrix type organic electro-luminescence display device may be divided into voltage programming type pixels, current programming type pixels and digital driving pixels.
FIG. 2 is a view showing a driving circuit of pixels PX arranged on the organic electro-luminescence display panel 40 of FIG. 1. As shown in FIG. 2, the driving circuit includes a switching transistor TFT21 driven by a scan signal SCAN applied via a scan line for transferring a data voltage VDATA applied via a data line to a storage capacitor C21, the storage capacitor C21 connected between a gate terminal of a driving transistor TFT22 and a terminal for a low power voltage Vss for charging the data voltage VDATA, the driving transistor TFT22 for supplying a driving current corresponding to the data voltage VDATA charged by the storage capacitor C21 to an organic light emitting diode OLED21, and the OLED21 having an anode connected to a terminal for a high power voltage VDD and a cathode connected to a drain of the driving transistor TFT22, for emitting light with a brightness corresponding to the driving current. Here, the transistors TFT21 and TFT22 may be implemented as N-channel thin film transistors (TFTs).
An operation of the related art pixel driving circuit having such configuration will be described with reference to FIG. 3.
The display controller 10 receives original video data provided from the exterior and a control signal for the data, thus to generate first and second timing signals TS1 and TS2. The display controller 10 then outputs the first timing signal TS1 and an image signal DATA to the data driving unit 20, and the second timing signal TS2 to the gate driving unit 30.
Positive scan signals SCAN1˜SCANn, as shown in FIG. 3, are sequentially supplied per every frame to the scan lines S1˜Sn on the electro-luminescence display panel 40 from the gate driving unit 30, and accordingly, the pixels PX on the corresponding scan lines (horizontal lines) are driven at each time of the supply. FIG. 2 illustrates one exemplary pixel among plural pixels PX (including a driving circuit) connected to an arbitrary scan line.
The switching transistor TFT21 is turned on by the corresponding scan signal among the scan signals SCAN1˜SCANn. Here, the data voltage VDATA supplied from the data driving unit 20 via the corresponding data line among the plural data lines D1˜Dm is charged in the storage capacitor C21 via the switching transistor TFT21, to thusly be maintained until before an emission period.
The driving transistor TFT22 is turned on by the data voltage VDATA charged in the storage capacitor C21, and accordingly a positive current corresponding to the data voltage VDATA flows via the OLED21, thus to allow the OLED21 to emit light with the corresponding brightness.
On the other hand, upon driving the organic electro-luminescence display panel 40 implemented as an amorphous silicon TFT (a-SI:H TFT), a threshold voltage Vth of the driving transistor TFT22 is shifted. In this case, the OLED21 does not normally emit light, causing a lowering of image quality. Such shift of the threshold voltage Vth may typically be caused by the data voltage VDATA applied to a gate node of the driving transistor TFT22 of the pixel driving circuit.
Hence, researches have recently been conducted to develop a technique for preventing the increase in the threshold voltage Vth in a manner of shifting a negative threshold voltage Vth by applying a negative voltage as well as the data voltage VDATA to the
As shown in FIG. 2, in the organic electro-luminescence pixel driving circuit including the two transistors TFT21 and TFT22 and the one storage capacitor C21, the OLED21 can be connected to an upper or lower end of the driving transistor TFT22.
One example of being connected to the upper end of the driving transistor TFT22 may include a Dual Plate OLED (DOD) structure. This structure is advantageous in that it is the simplest structure and also uses a Black Data Insertion (BDI) driving to effectively apply a negative voltage. Here, the BDI denotes that an emission-off interval is inserted in one frame in order to alleviate a TFT afterimage characteristic and improve a video image quality such as motion blur or the like.
However, in the related art organic electro-luminescence display device, upon applying a negative voltage to the gate node of the driving transistor, if a sufficient time was not given within one frame interval, the effect of preventing the increase in the threshold voltage was decreased.
In addition, since a driving data voltage relatively increased in order to enhance a deterioration compensation of the transistor, it was difficult to prevent the increase in the threshold voltage.